Directional solidification mould

ABSTRACT

A mould for use in directionally solidifying a component has a casting cavity including a single crystal selector spiral which is a helical passage having not less than 1¼ and not more than 1½ turns.

The present invention relates to a mould for use in directionally solidifying a component, and a pattern for forming such a mould.

Directional solidification can be used to produce cast components such as gas turbine blades. An advantage of a directionally solidified structure is that grain boundaries can be aligned along the length of a blade, providing increased creep strength at the high operating temperatures and stresses to which turbine blades may be exposed.

Creep strength can be increased still further by casting blades as single crystals in which there are no grain boundaries.

FIG. 1( a) shows schematically a cross-section through a directional solidification single crystal furnace. A ceramic-walled mould 1 is positioned on a copper chill plate 2, itself installed on a hydraulic or motor-driven ram 3. The furnace is then sealed and evacuated. When a predetermined level of vacuum is reached, the ram raises the mould into a resistance heated chamber 4 and the mould is allowed to soak. A charge of molten metal is poured into the mould using a pour cup 5.

FIG. 1( b) shows schematically a close up of the chill plate 2, and a base portion of the mould cavity.

The metal solidifies to form chill crystals 11 on the surface of the chill plate 2. The chill plate sets up a thermal gradient causing heat to flow in the direction from the heated chamber to the chill plate, and after a short period the chill crystals with the most favourable crystallographic orientations grow epitaxially as columnar grains 12 in the direction opposite to the heat flow direction to form a starter block 6 of solidified metal at the base portion of the mould cavity.

Above the base portion of the mould cavity, the mould has a grain selector spiral 7, which is a helical passage connecting the base portion of the mould cavity to the component portion 8 of the mould cavity. The crystals which grow fastest (generally those with the <001> direction aligned with the heat flow direction) are most likely to reach the entrance to the grain selector spiral first.

The spiral 7 acts as choke, reducing the number of crystals growing towards the exit of the spiral and the component portion 8 of the mould cavity above the spiral. When the process progresses correctly, a single crystal 13 of the desired crystallographic emerges from the spiral 6 into the component portion 8 of the mould cavity.

As the solidification front advances up the cavity, the mould is withdrawn via baffle plates 9 into a cooled lower chamber 10, thus maintaining the thermal gradient and the epitaxial growth. The mould continues to be withdrawn until the single crystal has grown the entire length and width of the component portion 8 of the mould cavity.

As a variant on this process, a pre-cast seed crystal can be located in the base portion of the mould cavity, the seed crystal already having the desired crystallographic orientation. Epitaxial growth on this crystal will then produce a suitably oriented grain. However, in practice, chill crystals tend to form at the melt back interface on the seed crystal, and these crystals can grow as fast or faster than the seed crystal itself. Thus, when a seed crystal is employed, it is still common to employ a selector spiral, or some non-helical variant thereof, to choke secondary grains.

Although the role of the selector spiral is particularly important when no seed crystal is used, even with a seed crystal it can be difficult to ensure that only one crystal enters the component portion of the mould cavity from the selector spiral. When two crystals enter, the result can be a bi-crystal component, or a component containing a high angle grain boundary. In either case, the component will generally have to be scrapped.

A first aspect of the present invention provides a mould for use in directionally solidifying a component, the mould having a casting cavity including a single crystal selector spiral, wherein the selector spiral is a helical passage having not less than 1¼ and not more than 1½ turns.

It has been found that by controlling the number of turns in the selector spiral to lie in the narrow range of from 1¼ to 1½ turns a surprising reduction in the scrap rate of directionally solidified single crystal components can be achieved.

It is believed that competing effects occur in the selector spiral. Increasing the number of turns increases the ability to choke off unwanted secondary grains. However, more turns also provide more surfaces for nucleation of secondary grains within the spiral itself. Experimentation has shown that the range of from 1¼ to 1½ turns provides an optimal balance between choking efficiency and limiting sites of secondary grain nucleation.

Preferably, the helical passage has a diameter of at least 4.5 mm, which helps to prevent the flow of molten metal into the mould being blocked at the spiral during bottom filling.

The component may be an aerofoil (such as a blade) of a gas turbine engine, and the casting cavity may further include a component cavity portion for forming the aerofoil, the selector spiral and the component cavity portion being in fluid communication. The selector spiral and the component cavity portion can then be arranged such that the axis of the selector spiral aligns with the stacking axis of the aerofoil when the aerofoil is formed in the component cavity portion. By aligning the spiral axis and the aerofoil component stacking axis, the single grain selected by the selector spiral can fill the component cavity portion starting from a central position in that cavity portion. This helps to reduce the likelihood of secondary grain nucleation and growth within the component cavity portion.

The mould can be adapted to be connectable to a second mould having a second casting cavity for forming the component, the selector spiral and the second casting cavity being in fluid communication when the moulds are thus connected. Typically the two moulds are formed from respective wax or plastic patterns which are coated and then eliminated. By separating the production of the pattern for the first mould from the production of the pattern for the second mould, it is easier to control quality and to avoid accidental damage to the part of the pattern corresponding to the spiral.

The first casting cavity may further include a starter block cavity portion in fluid communication with the selector spiral, the selector spiral and the starter block cavity portion being arranged such that, during directional solidification, growing crystals enter the selector spiral from the starter block cavity portion. Such a mould can be used, for example, with a chill plate positioned at a base end of the starter block cavity portion to form, chill crystals forming on the chill plate, the crystals growing with a columnar habit to form a starter block in the starter block cavity portion. Preferably, the starter block cavity portion is configured so that, when the selector spiral is positioned above the starter block cavity portion, a part of the starter block cavity portion extends vertically above the entrance to the selector spiral to provide a trap for debris in the cavity. That is, any debris in the trapped in the mould cavity may preferentially be washed or float into the trap, rather than into the selector spiral where it can initiate nucleation of secondary grains.

The mould may further have a seed crystal in fluid communication with the selector spiral, the selector spiral and the seed crystal being arranged such that, during directional solidification, the seed crystal grows and enters the selector spiral.

A second aspect of the invention provides a pattern for use in forming a mould according to the first aspect, the pattern being formed of material (such as wax or plastic) that can be eliminated to form the casting cavity of the mould when the pattern is coated with a material (such as ceramic) which forms the walls of the mould, and at least a portion of the pattern corresponding to the selector spiral. Thus that portion of the pattern corresponds to a helical passage having not less than 1¼ and not more than 1½ turns.

The pattern may have a cap portion extending from the part of the pattern corresponding to the entrance to the selector spiral, the cap portion being configured to cap, in use, a seed crystal or a further pattern corresponding to a starter block cavity portion. Thus the production of the pattern for the selector spiral can be separated from the production of the pattern for the starter block cavity portion or the seed crystal, which can make it is easier to control quality and to avoid accidental damage to the pattern for the spiral. For example, the cap portion may be positioned on the seed crystal or the further pattern only shortly prior to coating to form the mould walls.

The cap portion may be configured so that, when the portion of the pattern corresponding to the selector spiral is positioned above the cap portion, a part of the cap portion extends vertically above the part of the pattern corresponding to the entrance to the selector spiral, whereby said part of the cap portion corresponds to a trap for debris in the cavity of the mould.

The pattern may have an engagement portion which extends from a part of the pattern corresponding to the exit from the selector spiral for a growing crystal, in use, the pattern being joinable to a further pattern having a complementary engagement portion. After the two patterns are engaged they may be coated to form a mould. Typically the further pattern will be for forming a component cavity portion in the mould. The engagement portions can make it easier to join two separately produced patterns prior to coating. For example, the engagement portion and complementary engagement portion may be a male-female pair. Another aspect of the invention provides the combination of the two patterns.

A further aspect of the invention provides the use of the mould of the first aspect for directionally solidifying a component.

A further aspect of the invention provides the use of the pattern or patterns of the second aspect for forming a mould.

A further aspect of the invention provides a component moulded using the mould of the first aspect and still attached to material solidified in the single crystal selector spiral.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

FIG. 1 shows schematically a cross-section through a directional solidification single crystal furnace;

FIGS. 2( a) and (b) are respectively front and side schematic views of a pattern for a selector spiral;

FIG. 3 illustrates the use of a variant of the pattern of FIG. 2;

FIG. 4 illustrates the use of a further variant of the pattern of FIG. 2; and

FIG. 5 shows schematically a cross-section through a ceramic mould.

The embodiment of the invention described below may be used with either a seeded or non-seeded mould, although it has been conceived for use with a starter block (ie non-seeded).

FIGS. 2( a) and (b) are respectively front and side schematic views of a pattern 20 for a selector spiral. The pattern may be a plastics part, eg injection moulded polystyrene. The pattern has a helical part 21 with 1¼ to 1½ turns. At the bottom end of the helical part is a disc 22 which has a slightly conical upper surface. In use the disc is wax welded or glued to a pattern for the starter block (not shown). At the top end of the helical part 21 a curved extension 23 joins to an inverted elliptical frustocone 24, which is the connecting part to a wax pattern (not shown) of the component to be cast. A protrusion 25 at the top surface of the conical ellipse part is an engagement portion which couples to a corresponding recess in the component wax pattern in the manner of a male-female pair.

After connection of the selector spiral pattern to the starter block pattern and the component pattern, the complete pattern assembly is coated with ceramic layers which form the walls of the mould. The pattern assembly is then eliminated by heating, leaving a single crystal directional solidification mould, having a mould cavity including a helical passage selector spiral.

The 1¼ to 1½ turns of the selector spiral are sufficient to choke off secondary grains when the mould is used for single crystal directional solidification. However, the number of turns is not so high that secondary grain nucleation and growth in the selector spiral becomes excessive. Thus the selector spiral is effective at reducing bi-crystal growth in the component, and hence at reducing component scrap rates.

The component is typically a turbine blade of a gas turbine engine. The curved extension 23 allows the axis of the selector spiral to be aligned with the stacking axis of the turbine blade. This helps to reduce secondary grain nucleation in the component cavity of the mould.

FIG. 3 illustrates the use of a variant of the pattern of FIG. 2. In this variant, disc 22 is replaced by a cap 26 which can be positioned over the pattern for the start block 27, or indeed over a seed crystal. This can speed up pattern assembly and improve alignment of the selector spiral and the starter block or seed crystal.

FIG. 4 illustrates the use of a further variant of the pattern of FIG. 2. In this variant, the disc 22 is again replaced by a cap 28 positioned over the pattern for the starter block 27 or over a seed crystal. However, an annular part 28 of the cap now extends vertically above the part 29 of the pattern corresponding to the entrance to the selector spiral. In the mould, the annular part provides a gravity trap for buoyant debris that may be in the mould cavity. Trapping the debris in this location can prevent it entering the selector spiral where it may promote secondary grain nucleation.

FIG. 5 shows schematically a cross-section through a ceramic mould 30 which has a cavity comprising a single crystal selector spiral 31 (ie a helical passage having not less than 1¼ and not more than 1½ turns) and a starter block or seed crystal holding cavity portion 32 in fluid communication with the bottom end of the selector spiral. The mould may be formed, eg by an investment moulding process. The top of the mould can be joined to the bottom of a second mould which has a cavity for the component to be cast, ie so that the selector spiral and the component cavity are in fluid communication. Advantageously, this allows the process for forming the mould containing the selector spiral to be separated from the process for forming the mould for forming the component.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. 

1. A mould for use in directionally solidifying a component, the mould having a casting cavity including a single crystal selector spiral, wherein the selector spiral is a helical passage having not less than 1¼ and not more than 1½ turns.
 2. A mould according to claim 1, wherein the component is an aerofoil of a gas turbine engine, the casting cavity further includes a component cavity portion for forming the aerofoil, and the selector spiral and the component cavity portion are in fluid communication, the selector spiral and the component cavity portion being arranged such that the axis of the selector spiral aligns with the stacking axis of the aerofoil when the aerofoil is formed in the component cavity portion.
 3. A mould according to claim 1, wherein the mould is adapted to be connectable to a second mould having a second casting cavity for forming the component, the selector spiral and the second casting cavity being in fluid communication when the moulds are thus connected.
 4. A mould according to claim 1, wherein the first casting cavity further includes a starter block cavity portion in fluid communication with the selector spiral, the selector spiral and the starter block cavity portion being arranged such that, during directional solidification, growing crystals enter the selector spiral from the starter block cavity portion.
 5. A mould according to claim 4, wherein the starter block cavity portion is configured so that, when the selector spiral is positioned above the starter block cavity portion, a part of the starter block cavity portion extends vertically above the entrance to the selector spiral to provide a trap for debris in the cavity.
 6. A mould according to claim 1, wherein the mould further has a seed crystal in fluid communication with the selector spiral, the selector spiral and the seed crystal being arranged such that, during directional solidification, the seed crystal grows and enters the selector spiral.
 7. A pattern for use in forming a mould according to claim 1, the pattern being formed of material that can be eliminated to form the casting cavity of the mould when the pattern is coated with a material which forms the walls of the mould, and at least a portion of the pattern corresponding to the selector spiral.
 8. A pattern according to claim 7, wherein the pattern has a cap portion extending from the part of the pattern corresponding to the entrance to the selector spiral, the cap portion being configured to cap, in use, a seed crystal or a further pattern corresponding to a starter block cavity portion.
 9. A pattern according to claim 8, wherein the cap portion is configured so that, when the portion of the pattern corresponding to the selector spiral is positioned above the cap portion, a part of the cap portion extends vertically above the part of the pattern corresponding to the entrance to the selector spiral, whereby said part of the cap portion corresponds to a trap for debris in the cavity of the mould.
 10. A pattern according to claim 7, wherein the pattern has an engagement portion which extends from a part of the pattern corresponding to the exit from the selector spiral for a growing crystal, in use, the pattern being joinable to a further pattern having a complementary engagement portion.
 11. The combination of a pattern according to claim 10 and the further pattern.
 12. A method for directionally solidifying a component with the mould of claim
 1. 13. A method for forming a mould with the pattern or patterns of claim
 7. 14. A component moulded using the mould of claim 1 and still attached to material solidified in the single crystal selector spiral. 